Probe cards are typically used in the testing of integrated circuit (IC) devices. Due to their design, probe cards are particularly advantageous for testing entire semiconductor wafers to detect any manufacturing defects before they are diced and packaged. For example, a probe card is typically formed from a printed circuit board (PCB) having a number of electrical contact elements and/or traces disposed thereon to connect to a testing apparatus. The PCB is connected to a probe head having a number of pins that are brought into contact with a device under test (DUT) to facilitate the transmission of electrical signals to and from the DUT. Accordingly, the probe card acts as an interface between the testing apparatus and the DUT.
Because the probe head serves as the primary interface with the DUT, the pitch (i.e., spacing between the pins) of the probe head must be very small in order to properly align with corresponding contact pads of the DUT. On the other hand, the electrical traces on the PCB are generally coarser and spaced further apart to be more easily connected to the testing apparatus (e.g., automatic test equipment or “ATE”). Accordingly, many probe cards additionally include a space transformer disposed between the PCB and the probe head to interface the pins of the probe head with the electrical traces on the PCB. A typical space transformer is made of a multilayer ceramic material having a plurality of transmission paths formed therein to connect the probe head to the PCB. Such space transformers can be very expensive to produce. In contrast, a low-cost space transformer is made up of a number of wires that form the transmission paths connecting the probe head to the PCB. However, the lengths of the transmission paths can have many adverse effects on the signals communicated to and from the DUT. For example, in high frequency signaling (where a switching edge of an electrical signal is short relative to the length of the transmission path), any slight discontinuities in impedance along the length of the transmission path will create reflections, thus causing the transmitted signal to become distorted. In addition, most IC devices must be powered (e.g., by receiving a power signal) in order to function. However, because longer ground paths also have greater inductances, a long power path will radiate and be more susceptible to external noise and interference.
As die sizes continue to shrink, so too does the pitch of the contact pads of IC devices. Accordingly, there is a need for a probe card that can be used in the testing of such fine pitch IC devices. More specifically, there is a need for a low cost means of interfacing the pins of a fine pitch probe head with corresponding contacts on a PCB, without sacrificing signal quality or efficiency.
Like reference numerals refer to corresponding parts throughout the drawing figures.